Research On Degradation And Mechanical Properties Of Poly(L-Lactic Acid)Bone Scaffolds Fabricated By Laser Additive Manufacturing

Poly（l-lacti acid）（PLLA）with good biocompatibility and biodegradability exhibits great potential in the repair of bone defects.However,PLLA bone scaffold possesses slow degradation rate and inadequate mechanical properties,which limits the effect of bone defect repair.In this paper,L-lactide（L-LA）was blended with PLLA,and the PLLA/L-LA scaffold was fabricated by selective laser sintering（SLS）.The introduction of L-LA decreased the crystallinity of PLLA scaffold,accelerating its degradation.Carbon fiber（CF）modified by nano-Si O₂particles was used to enhance the mechanical properties of PLLA scaffold by constructing interfacial crystallization structure and mechanical locking.Biosoluble ceramic fiber（BCF）was used to accelerate the degradation of PLLA scaffold utilizing its solubility and enhance the mechanical properties by the pull-out,fracture and debonding effects of BCF.The main work and innovations of this paper are as follows:1.PLLA scaffolds with internally interconnected pores and customizable external shapes were fabricated by SLS.The effects of processing parameters including laser power,scanning mode,scanning speed,scanning distance and layering thickness on scaffold properties were studied,and the optimum processing parameters were:laser power of 2 W,progressive scanning,scanning speed of 100 mm/s,scanning distance of0.25 mm and layer thickness of 200-300μm.2.L-LA was used to enhance the hydrophilicity and decrease the crystallinity of PLLA scaffold to accelerate the degradation rate.It could be found that the introduction of L-LA enhanced the hydrophilia of scaffold by increasing the content of hydrophilic functional groups（-COOH and-OH）.What’s more,L-LA as the dimer of PLLA possesses good compatibility with PLLA.It could weaken the interaction force among PLLA chains,hindering the ordered arrangement of PLLA molecular chains in the crystallization process,increasing the amorphous region and decreasing the crystallinity of PLLA scaffold.The immersion tests indicated that the PLLA/20L-LA possessed the maximal degradation rate,and the weight loss reached 4.90%and 8.25%after soaking for 2 and 4weeks,respectively.3.CF modified by nano-Si O₂ particles was used to enhance the mechanical properties of PLLA scaffold by constructing interfacial crystallization structure and mechanical locking.The carboxyl groups on CF and hydroxyl groups on nano-Si O₂ could react with silanol groups in silane coupling agents KH550 and KH560,respectively.The amino and epoxy groups were introduced on the surface of CF and Si O₂,respectively.And nano-Si O₂ was successfully grafted on CF surface because epoxy groups were more reactive and easily reacted to the amino groups.The reinforcement mechanisms of mechanical properties were:on the one hand,nano-Si O₂ induced PLLA chains to form crystal nuclei which grew perpendicular to the axis of CF,forming transcrystalline structure and enhancing the interfacial bonding.On the other hand,nano-Si O₂ particles increased the roughness of CF,forming strong mechanical locking between PLLA and CF,enhancing the mechanical properties of PLLA scaffold.Compared to the PLLA scaffold,the strength and modulus of composite scaffold increased by 40.6%and 37.2%,respectively.4.BCF was used to accelerate the degradation rate by constructing an alkaline microenvironment and water diffusion channels and enhance the mechanical properties by the pull-out,fracture and debonding effects.It could be found that BCF could be dissolved and produce Ca²⁺,Mg²⁺and OH^-to form an alkaline microenvironment,neutralizing the acid products after degradation and promoting the hydrolysis of PLLA.With the diffusion of water and dissolution of BCF,water diffusion channels formed along BCF,speeding up the diffusion of water and accelerating the degradation of PLLA scaffold.After degradation for 4 weeks,the PLLA/20BCF scaffold had the largest weight loss rate of 4.02%,about 6times that of the PLLA scaffold.Meanwhile,when the PLLA scaffold was stressed,BCF could bear partial load transferred from the matrix,which hinder the deformation and fracture of PLLA matrix.Pull-out,fracture and debonding of BCF could absorb partial fracture energy,retarding the propagation of crack and enhancing the mechanical properties of PLLA scaffold.The tensile and compressive strength of the PLLA/15BCF scaffold was the highest,which were 52.85%and 71.02%higher than that of the PLLA scaffold,respectively.